1. Field of the Invention
The present invention relates to an optical head assembly for use in an optical disk driver using light having wavelengths different from each other for at least two types of optical disks, and more particularly, to an optical head assembly for use in an optical disk driver which is compatible with a digital versatile disk (DVD) and a recordable compact disk (CD-R)
2. Description of the Related Art
In an optical disk apparatus using a recording medium for storing a large quantity of information, a compact disk (CD) and a digital versatile disk (DVD) have been widely used. Recently, a recordable compact disk (CD-R) and a digital versatile disk--random access memory (DVD-RAM) have been known. When reproducing information, laser light having a wavelength of 780 nm is used for a CD-R, and laser light having a wavelength of 650 nm or 635 nm is used for a DVD-RAM.
Accordingly, an optical disk driver which can record information on and reproduce information from these optical disks, includes at least two light sources for emitting laser light having different wavelengths from each other. Referring to FIG. 1, description on an optical system of an optical head for an optical disk driver which is compatible with a conventional DVD and a CD-R is as follows.
FIG. 1 shows a conventional optical head assembly for use in an optical disk driver for a digital versatile disk (DVD) and a recordable compact disk (CD-R). The optical head assembly includes a first light source 1 for emitting a first light beam having a wavelength of 635 nm for recording and reproducing information with respect to a digital versatile disk (DVD) 8, a second light source 11 for emitting a second light beam having a wavelength of 780 nm for recording and reproducing information with respect to a recordable compact disk (CD-R) 9, and an objective lens 7 for focusing the first and second light beams emitted from the first and second light sources 1 and 11 on an information recording surface of the DVD 8 and that of the CD-R 9, respectively.
The first light beam emitted from the first light source 1 is collimated in the form of a parallel beam by a first collimating lens 2 and then is transmitted to a first beam splitter 3. The first beam splitter 3 directs the first light beam incident from the first collimating lens 2 to an interference filtering prism 4, and transmits the first light beam returned from the interference filtering prism 4 to a first photodetector 10. In more detail, the first beam splitter 3 directs the first light beam, which is a P polarization beam incident from the first collimating lens 2, to the interference filtering prism 4, and transmits the first light beam which is an S polarization beam incident from the interference filter prism 4 to the first photodetector 10.
A variable aperture 6 having a thin film structure transmits the first light beam, which is a parallel light beam incident from a quarter-wave plate 5 to the objective lens 7. The objective lens 7 focuses the first light beam passed through the variable aperture 6 on an information recording surface of the DVD 8 having a thickness of 0.6 mm.
Therefore, the first light beam is focussed on the information recording surface of the DVD 8 by the objective lens 7, and the focussed first light beam reflected from the information recording surface of the DVD 8 contains the information recorded at a position on which the first light beam is focussed. The reflected first light beam passes through the objective lens 7, the variable aperture 6, the quarter-wave plate 5, the interference filter prism 4 and the first beam splitter 3, in sequence, and then is received by the first photodetector 10. The first photodetector 10 detects information from the first light beam incident from the first beam splitter 3.
The second light beam of the 780 nm wavelength which is emitted from the second light source 11 passes through a second collimating lens 12 and a second beam splitter 13, and then is incident to a convergent lens 14.
The convergent lens 14 converges the second light beam incident from the second beam splitter 13 to then transmit the converged second light beam to the interference filter prism 4. The interference filter prism 4, which totally transmits or reflects incident light according to a wavelength, totally transmits the first light beam for the DVD having a wavelength of 635 nm and totally reflects the second light beam having a wavelength of 780 nm incident from the convergent lens 14.
The interference filter prism 4 reflects the second light beam incident from the convergent lens 14 to the quarter-wave plate 5 in the form of a divergent beam. The quarter-wave plate 5 transmits the second light beam incident from the interference filter prism 4 to the variable aperture 6. The variable aperture 6 passes only part of the second light beam of the 780 nm wavelength incident in the form of a divergent beam. The objective lens 7 focuses the second light beam incident from the variable aperture 6 on the information recording surface of the CD-R 9 having a thickness of 1.2 mm. Thus, the second light beam is focussed on the information recording surface of the CD-R 9 by the objective lens 7, and the second light beam reflected by the information recording surface contains information recorded at a position on which the second light beam is focussed.
The second beam splitter 13 of FIG. 1 makes the second light beam incident from the convergent lens 14 proceed to a second photodetector 15. In more detail, the second beam splitter 13 totally transmits the second light beam incident from the second collimating lens 12 to the convergent lens 14, and totally reflects the second light beam incident from the convergent lens 14 to the second photodetector 15. The second photodetector 15 receives the second light beam from the second beam splitter 13 and detects information from the second light beam.
FIG. 2 shows in detail the variable aperture 6 shown in FIG. 1 having a thin film structure. The variable aperture 6 having a thin film structure, as shown in FIG. 2, can selectively transmit the rays of the light incident to the region not more than the numerical aperture (NA) of 0.6. Among two regions, a first region 1 has the numerical aperture (NA) of 0.45 or below and transmits both 780 nm and 635 nm wavelength lights. A second region 2 shown is made by coating a dielectric thin film and has the numerical aperture (NA) of 0.45 or more, and totally transmits the 635 nm wavelength light and totally reflects the 780 nm wavelength light. The region 1 is comprised of a quartz (SiO.sub.2) thin film in order to remove any optical aberration generated by the dielectric thin film coated region 2. By using the variable aperture 6, the 780 nm wavelength light passing through the region 1 having the 0.45 NA or below forms a beam spot appropriate to the CD-R on the signal recording surface thereof. The 635 nm wavelength light transmitted through the regions 1 and 2 having the 0.6 NA or below forms a beam spot appropriate to the DVD on the signal recording surface thereof.
As shown, the optical head system of FIG. 1 can perform recording and reproduction operations with respect to both a DVD and a CD-R.
In the above optical head system, all optical systems for compatibility with the DVD and the CD-R are constructed as a single unit, and thus an actuator carrying the unit is imposed under more load. Accordingly, a data access time and a target sector seek time increase.